Toroidal low-field reactive gas source

ABSTRACT

An apparatus for dissociating gases includes a plasma chamber that may be formed from a metallic material and a transformer having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. The apparatus also includes one or more switching semiconductor devices that are directly coupled to a voltage supply and that have an output coupled to the primary winding of the transformer. The one or more switching semiconductor devices drive current in the primary winding that induces a potential inside the chamber that forms a plasma which completes a secondary circuit of the transformer.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/659,881,filed on Sep. 12, 2000, Now U.S. Pat. No. 6,886,431 which is acontinuation of U.S. patent application Ser. No. 08/883,281 filed onJun. 26, 1997, now U.S. Pat. No. 6,150,628.

FIELD OF THE INVENTION

This invention relates generally to the field of generating activatedgas containing ions, free radicals, atoms and molecules and to apparatusfor and methods of processing materials with activated gas.

BACKGROUND OF THE INVENTION

Plasma discharges can be used to excite gases to produce activated gasescontaining ions, free radicals, atoms and molecules. Activated gases areused for numerous industrial and scientific applications includingprocessing solid materials such as semiconductor wafers, powders, andother gases. The parameters of the plasma and the conditions of theexposure of the plasma to the material being processed vary widelydepending on the application.

For example, some applications require the use of ions with low kineticenergy (i.e. a few electron volts) because the material being processedis sensitive to damage. Other applications, such as anisotropic etchingor planarized dielectric deposition, require the use of ions with highkinetic energy. Still other applications, such as reactive ion beametching, require precise control of the ion energy.

Some applications require direct exposure of the material beingprocessed to a high density plasma. One such application is generatingion-activated chemical reactions. Other such applications includeetching of and depositing material into high aspect ratio structures.Other applications require shielding the material being processed fromthe plasma because the material is sensitive to damage caused by ions orbecause the process has high selectivity requirements.

Plasmas can be generated in various ways including DC discharge, radiofrequency (RF) discharge, and microwave discharge. DC discharges areachieved by applying a potential between two electrodes in a gas. RFdischarges are achieved either by electrostatically or inductivelycoupling energy from a power supply into a plasma. Parallel plates aretypically used for electrostatically coupling energy into a plasma.Induction coils are typically used for inducing current into the plasma.Microwave discharges are achieved by directly coupling microwave energythrough a microwave-processing window into a discharge chambercontaining a gas. Microwave discharges are advantageous because they canbe used to support a wide range of discharge conditions, includinghighly ionized electron cyclotron resonant (ECR) plasmas.

RF discharges and DC discharges inherently produce high energy ions and,therefore, are often used to generate plasmas for applications where thematerial being processed is in direct contact with the plasma. Microwavedischarges produce dense, low ion energy plasmas and, therefore, areoften used to produce streams of activated gas for “downstream”processing. Microwave discharges are also useful for applications whereit is desirable to generate ions at low energy and then accelerate theions to the process surface with an applied potential.

However, microwave and inductively coupled plasma sources requireexpensive and complex power delivery systems. These plasma sourcesrequire precision RF or microwave power generators and complex matchingnetworks to match the impedance of the generator to the plasma source.In addition, precision instrumentation is usually required to ascertainand control the actual power reaching the plasma.

RF inductively coupled plasmas are particularly useful for generatinglarge area plasmas for such applications as semiconductor waferprocessing. However, prior art RF inductively coupled plasmas are notpurely inductive because the drive currents are only weakly coupled tothe plasma. Consequently, RF inductively coupled plasmas are inefficientand require the use of high voltages on the drive coils. The highvoltages produce high electrostatic fields that cause high energy ionbombardment of reactor surfaces. The ion bombardment deteriorates thereactor and can contaminate the process chamber and the material beingprocessed The ion bombardment can also cause damage to the materialbeing processed.

Faraday shields have been used in inductively coupled plasma sources tocontain the high electrostatic fields. However, because of therelatively weak coupling of the drive coil currents to the plasma, largeeddy currents form in the shields resulting in substantial powerdissipation. The cost, complexity, and reduced power efficiency make theuse of Faraday shields unattractive.

SUMMARY OF THE INVENTION

It is therefore a principle object of this invention to provide a sourceof activated gas that uses a high efficiency RF power coupling devicewhich couples power into a plasma without the use of conventional RF ormicrowave generators and impedance matching systems.

It is another principle object of this invention to provide a source ofactivated gas for materials processing where there is no significantenergetic ion bombardment within the process reactor and where long-termoperation can be sustained using chemically reactive gases withoutdamage to the source and without production of contaminant materials.

It is another principle object of this invention to provide a source ofactivated gas in which either a metal, a dielectric, or a coated metal(e.g. anodized) can be used to form the source chamber.

A principal discovery of the present invention is that switchingsemiconductor devices can be used to efficiently drive the primarywinding of a power transformer that couples electromagnetic energy to aplasma so as to form a secondary circuit of the transformer. It isanother principal discovery of this invention that an inductively-driventoroidal plasma source can be constructed with a metallic plasmachamber.

Accordingly, the present invention features an apparatus fordissociating gases that includes a plasma chamber. The plasma chambermay be formed from a metallic material such as aluminum or may be formedfrom a dielectric material such as quartz. The metallic material may bea refractory metal. The apparatus may include a process chamber that iscoupled to the plasma chamber and positioned to receive reactive gasgenerated by a plasma in the plasma chamber.

The apparatus also includes a transformer having a magnetic coresurrounding a portion of the plasma chamber and having a primarywinding. One or more switching semiconductor devices are directlycoupled to a voltage supply and have an output coupled to the primarywinding of the transformer. The output of the one or more switchingsemiconductor devices may be directly coupled to the primary winding ofthe transformer. The one or more switching semiconductor devices may beswitching transistors. The voltage supply may be a line voltage supplyor a bus voltage supply.

The apparatus may include a free charge generator which assists theignition of a plasma in the chamber. In a preferred embodiment, anelectrode is positioned in the chamber to generate the free charges. Inanother preferred embodiment, an electrode is capacitively coupled tothe chamber to generate the free charges. In another preferredembodiment, an ultraviolet light source is optically coupled to thechamber to generate the free charges.

The apparatus may include a circuit for measuring electrical parametersof the primary winding and of the plasma. The circuit measuresparameters such as the current driving the primary winding, the voltageacross the primary winding, the bus supply voltage, the average power inthe primary winding, and the peak power in the primary winding. A powercontrol circuit may be coupled to the circuit for measuring electricalparameters of the primary winding and the plasma. The power controlcircuit regulates the current flowing through the primary windings basedupon a measurement of the electrical properties of the primary windingand of the plasma and from a predetermined set point representing adesired operating condition.

The present invention also features a method for dissociating gases. Themethod includes providing a chamber for containing a gas at a pressure.The pressure may be substantially between 1 mtorr and 100 torr. The gasmay comprise a noble gas, a reactive gas or a mixture of at least onenoble gas and at least one reactive gas. The method also includesproviding a transformer having a magnetic core surrounding a portion ofthe chamber and having a primary winding.

In addition, the method includes directly coupling one or more switchingsemiconductor devices to a voltage supply, which may be a line voltagesupply or a bus voltage supply. The one or more switching semiconductordevices are also coupled to the primary winding of the transformer sothat they generate a current that drives the primary winding. The one ormore switching semiconductor devices may be directly coupled to theprimary winding of the transformer.

The method also includes inducing a potential inside the plasma chamberwith the current in the primary winding of the transformer. Themagnitude of the induced potential depends on the magnetic fieldproduced by the core and the frequency at which the switchingsemiconductor devices operate according to Faraday's law of induction.The potential forms a plasma which completes a secondary circuit of thetransformer. The electric field of the plasma may be substantiallybetween 1-100 V/cm. The method may include providing an initialionization event in the chamber. The initial ionization event may be theapplication of a voltage pulse to the primary winding or to an electrodepositioned in the plasma chamber. The initial ionization event may alsobe exposing the chamber to ultraviolet radiation.

The method may include the step of measuring electrical parameters ofthe primary winding and of the plasma including one or more of thecurrent driving the primary winding, the voltage across the primarywinding, the bus voltage, the average power in the primary winding, andthe peak power in the primary winding. In addition, the method mayinclude the step of determining an output of the one or more switchingsemiconductor devices from the measurement of the electrical parametersof the primary winding, the plasma, and from a predetermined set pointrepresenting a desired operating condition.

The present invention also includes a method for cleaning a processchamber. The method includes providing a plasma chamber that is coupledto the process chamber. The plasma chamber contains a reactive gas at apressure. A transformer is provided having a magnetic core surrounding aportion of the plasma chamber and having a primary winding. One or moreswitching semiconductor devices is directly coupled to a voltage supplyfor generating a current that drives the primary winding of thetransformer.

In addition, the method includes inducing a potential inside the plasmachamber with the current in the primary winding. The magnitude of theinduced potential depends on the magnetic field produced by the core andthe frequency at which the switching semiconductor devices operateaccording to Faraday's law of induction. The potential forms a plasmawhich completes a secondary circuit of the transformer. The method alsoincludes directing chemically active species such as atoms, moleculesand radicals generated in the plasma from the plasma chamber into theprocess chamber thereby cleaning the process chamber.

The present invention also includes a method for generating reactivegases. The method includes providing a plasma chamber that is coupled tothe process chamber. The plasma chamber contains a reactive gas at apressure. A transformer is provided having a magnetic core surrounding aportion of the plasma chamber and having a primary winding. One or moreswitching semiconductor devices is directly coupled to a voltage supplyfor generating a current that drives the primary winding of thetransformer.

In addition, the method includes inducing a potential inside the plasmachamber with the current in the primary winding. The magnitude of theinduced potential depends on the magnetic field produced by the core andthe frequency at which the switching semiconductor devices operateaccording to Faraday's law of induction. The potential forms a plasmawhich completes a secondary circuit of the transformer. The method alsoincludes generating reactive gas in the plasma.

The present invention also features an apparatus for generating ions.The apparatus includes a plasma chamber that may be formed from ametallic material such as a refractory metal. An orifice is positionedin the chamber for directing ions generated by the plasma. A processchamber may be coupled to the orifice in the plasma chamber and adaptedto receive ions generated by the plasma. Accelerating electrodes may bepositioned in the process chamber for accelerating ions generated by theplasma.

The apparatus includes a transformer having a magnetic core surroundinga portion of the plasma chamber and having a primary winding. One ormore switching semiconductor devices are directly coupled to a voltagesupply, which may be a line voltage supply or a bus voltage supply, andhave an output coupled to the primary winding of the transformer. Inoperation, the one or more switching semiconductor devices drivescurrent in the primary winding of the transformer. The current induces apotential inside the chamber that forms a plasma which completes asecondary circuit of the transformer. Ions are extracted from the plasmathrough the orifice. The ions may be accelerated by the acceleratingelectrodes.

The present invention also features another apparatus for dissociatinggases. The apparatus includes a plasma chamber that comprises anelectrically conductive material such as aluminum and at least onedielectric region that prevents induced current flow in the chamber. Theplasma chamber may include a plurality of dielectric regions separatingat least two regions of the plasma chamber. The dielectric region maycomprise a dielectric coating on at least one mating surface of thechamber. The plasma chamber may also include cooling channels forpassing a fluid that controls the temperature of the chamber.

In addition, the apparatus includes a transformer having a magnetic coresurrounding a portion of the plasma chamber and having a primarywinding. The apparatus also includes a power supply that has an outputelectrically connected to the primary winding of the transformer. Thepower supply drives current in the primary winding that induces apotential inside the chamber that forms a plasma which completes asecondary circuit of the transformer. The power supply may comprise oneor more switching semiconductor devices that are directly coupled to avoltage supply and that have an output coupled to the primary winding ofthe transformer. The voltage supply may comprise a line voltage supplyor a bus voltage supply.

The apparatus may include a means for generating free charges thatassists the ignition of a plasma in the chamber. In a preferredembodiment, an electrode is positioned in the chamber to generate thefree charges. In another preferred embodiment, an electrode iscapacitively coupled to the chamber to generate the free charges. Inanother preferred embodiment, an ultraviolet light source is opticallycoupled to the chamber to generate the free charges.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is described with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a toroidal low-field plasmasource for producing activated gases that embodies the invention.

FIG. 2 illustrates a plot of etch rate of thermal silicon dioxide as afunction of NF3 feed gas flow rate, using the toroidal low-field plasmasource that embodies the invention.

FIG. 3 is a schematic representation of a metallic plasma chamber thatmay be used with the toroidal low-field plasma source described inconnection with FIG. 1.

FIG. 4 is a schematic representation of a dielectric spacer suitable forthe dielectric regions illustrated in FIG. 3 that prevent inducedcurrent flow from forming in the plasma chamber.

FIG. 5 is a schematic representation of a toroidal low-field ion beamsource that embodies the invention and that is configured for highintensity ion beam processing.

FIG. 6 is a schematic block diagram of a solid state switching powersupply that includes the one or more switching semiconductor devices ofFIG. 1.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of a toroidal low-field plasmasource 10 for producing activated gases that embodies the invention. Thesource 10 includes a power transformer 12 that couples electromagneticenergy into a plasma 14. The power transformer 12 includes a highpermeability magnetic core 16, a primary coil 18, and a plasma chamber20 which allows the plasma 14 to form a secondary circuit of thetransformer 12. The power transformer 12 can include additional magneticcores and conductor primary coils (not shown) that form additionalsecondary circuits.

The plasma chamber 20 may be formed from a metallic material such asaluminum or a refractory metal, or may be formed from a dielectricmaterial such as quartz. One or more sides of the plasma chamber 20 maybe exposed to a process chamber 22 to allow charged particles generatedby the plasma 14 to be in direct contact with a material to be processed(not shown). A sample holder 23 may be positioned in the process chamber22 to support the material to be processed. The material to be processedmay be biased relative to the potential of the plasma.

A voltage supply 24, which may be a line voltage supply or a bus voltagesupply, is directly coupled to a circuit 26 containing one or moreswitching semiconductor devices. The one or more switching semiconductordevices may be switching transistors. The circuit may be a solid stateswitching power supply. An output 28 of the circuit 26 may be directlycoupled to the primary winding 18 of the transformer 12.

The toroidal low field plasma source 10 may include a means forgenerating free charges that provides an initial ionization event thatignites a plasma in the plasma chamber 20. The initial ionization eventmay be a short, high voltage pulse that is applied to the plasmachamber. The pulse may have a voltage of approximately 500-10,000 voltsand may be approximately 0.1 to 10 microseconds long. A noble gas suchas argon may be inserted into the plasma chamber 20 to reduce thevoltage required to ignite a plasma. Ultraviolet radiation may also beused to generate the free charges in the plasma chamber 20 that providethe initial ionization event that ignites the plasma in the plasmachamber 20.

In a preferred embodiment, the short, high voltage electric pulse isapplied directly to the primary coil 18 to provide the initialionization event. In another preferred embodiment, the short, highvoltage electric pulse is applied to an electrode 30 positioned in theplasma chamber 20. In another preferred embodiment, the short, highvoltage electric pulse is applied to an electrode 32 that iscapacitively coupled to the plasma chamber 20 by a dielectric. Inanother preferred embodiment, the plasma chamber 20 is exposed toultraviolet radiation emitting from an ultraviolet light source 34 thatis optically coupled to the plasma chamber 20. The ultraviolet radiationcauses the initial ionization event that ignites the plasma.

The toroidal low field plasma source 10 may also include a circuit 36for measuring electrical parameters of the primary winding 18.Electrical parameters of the primary winding 18 include the currentdriving the primary winding 18, the voltage across the primary winding18, the bus or line voltage supply generated by the voltage supply 24,the average power in the primary winding 18, and the peak power in theprimary winding 18.

In addition, the plasma source 10 may include a means for measuringrelevant electrical parameters of the plasma 14. Relevant electricalparameters of the plasma 14 include the plasma current and power. Forexample, the source 10 may include a current probe 38 positioned aroundthe plasma chamber 20 to measure the plasma current flowing in secondaryof the transformer 12. The plasma source 10 may also include an opticaldetector 40 for measuring the optical emission from the plasma 14. Inaddition, the plasma source 10 may include a power control circuit 42that accepts data from one or more of the current probe 38, the powerdetector 40, and the circuit 26 and then adjusts the power in the plasmaby adjusting the current in the primary winding 18.

In operation, a gas is bled into the plasma chamber 20 until a pressuresubstantially between 1 mtorr and 100 torr is reached. The gas maycomprise a noble gas, a reactive gas or a mixture of at least one noblegas and at least one reactive gas. The circuit 26 containing switchingsemiconductor devices supplies a current to the primary winding 18 thatinduces a potential inside the plasma chamber. The magnitude of theinduced potential depends on the magnetic field produced by the core andthe frequency at which the switching semiconductor devices operateaccording to Faraday's law of induction. An ionization event that formsthe plasma may be initiated in the chamber. The ionization event may bethe application of a voltage pulse to the primary winding or to theelectrode 30 in the chamber 20. Alternatively, the ionization event maybe exposing the chamber to ultraviolet radiation.

Once the gas is ionized, a plasma is formed which completes a secondarycircuit of the transformer. The electric field of the plasma may besubstantially between 1-100 V/cm. If only noble gases are present in theplasma chamber 20, the electric fields in the plasma 14 may be as low as1 volt/cm. If, however, electronegative gases are present in thechamber, the electric fields in the plasma 14 are considerably higher.Operating the plasma source 10 with low electric fields in the plasmachamber 14 is desirable because a low potential difference between theplasma and the chamber will substantially reduce erosion of the chamberby energetic ions and the resulting contamination to the material beingprocessed.

The power delivered to the plasma can be accurately controlled by afeedback loop 44 that comprises the power control circuit 42, thecircuit 36 for measuring electrical parameters of the primary winding 18and the circuit 26 containing one or more switching semiconductordevices. In addition, the feedback loop 44 may include the current probe38 and optical detector 40.

In a preferred embodiment, the power control circuit 42 measures thepower in the plasma using the circuit 36 for measuring electricalparameters of the primary winding 18. The power control circuit 42 thencompares the measurement to a predetermined setpoint representing adesired operating condition and adjusts one or more parameters of thecircuit 26 to control the power delivered to the plasma. The one or moreparameters of circuit 26 include pulse amplitude, frequency, pulsewidth, and relative phase of the drive pulses to the one or moreswitching semiconductor devices.

In another preferred embodiment, the power control circuit 42 measuresthe power in the plasma using the current probe 38 or the opticaldetector 40. The power control circuit 42 then compares the measurementto a predetermined setpoint representing a desired operating conditionand adjusts one or more parameters of the circuit 26 to control thepower delivered to the plasma.

The plasma source 10 is advantageous because its conversion efficiencyof line power into power absorbed by the plasma is very high comparedwith prior art plasma sources. This is because the circuit 26 containingone or more switching semiconductor devices that supplies the current tothe primary winding 18 is highly efficient. The conversion efficiencymay be substantially greater than 90%. The plasma source 10 is alsoadvantageous because it does not require the use of conventionalimpedance matching networks or conventional RF power generators. Thisgreatly reduces the cost and increases the reliability of the plasmasource.

In addition, the plasma source 10 is advantageous because it operateswith low electric fields in the plasma chamber 20. Low electric fieldsare desirable because a low potential difference between the plasma andthe chamber will substantially reduce energetic ion bombardment withinthe plasma chamber 20. Reducing energetic ion bombardment in the plasmachamber 20 is desirable because it minimizes the production ofcontaminating materials within the plasma chamber 20, especially whenchemically reactive gases are used. For example, when fluorine basedgases such as NF3 and CF4/02 are used in the plasma source 10 of thepresent invention, including a plasma chamber formed from a fluorineresistant material, no or minimal erosion of the chamber was observedafter extended exposure to the low ion temperature fluorine plasma.

The plasma source 10 is useful for processing numerous materials such assolid surfaces, powders, and gases. The plasma source 10 is particularlyuseful for cleaning process chambers in semiconductor processingequipment such as thin film deposition and etching systems. The plasmasource 10 is also particularly useful for providing an ion source forion implantation and ion milling systems.

In addition, the plasma source 10 is useful for providing a source foretching systems used for etching numerous materials used to fabricatesemiconductor devices such as silicon, silicon dioxide, silicon nitride,aluminum, molybdenum, tungsten and organic materials such asphotoresists, polyimades and other polymeric materials. The plasmasource 10 is also useful for providing a source for plasma enhanceddeposition of materials of numerous thin films such as diamond films,silicon dioxide, silicon nitride, and aluminum nitride.

The plasma source is also useful for generating reactive gases such asatomic fluorine, atomic chlorine and atomic oxygen. Such reactive gasesare useful for reducing, converting, stabilizing or passivating variousoxides such as silicon dioxide, tin oxide, zinc oxide and indium-tinoxide. Applications include fluxless soldering, removal of silicondioxide from silicon surface and passivation of silicon surface prior towafer processing.

Other applications of the plasma source 10 include modification ofsurface properties of polymers, metals, ceramics and papers. The plasmasource 10 may also be used for abatement of environmentally hazardousgases including fluorine containing compounds such as CF4, NF3, C2F6,CHF3, SF6, and organic compounds such as dioxins and furans and othervolatile organic compounds. In addition, the plasma source 10 may beused to generate high fluxes of atomic oxygen, atomic chlorine, oratomic fluorine for sterilization. The plasma source 10 may also be usedin an atmospheric pressure torch.

FIG. 2 illustrates a plot of etch rate of thermal silicon dioxide as afunction of NF3 feed gas flow rates using the toroidal low-field plasmasource that embodies the invention. The toroidal low-field plasma source10 was configured as a downstream atomic fluorine source. The power wasapproximately 3.5 kW.

FIG. 3 is a schematic representation of a metallic plasma chamber 100that may be used with the toroidal low-field plasma source described inconnection with FIG. 1. The plasma chamber 100 is formed from a metalsuch as aluminum, copper, nickel and steel. The plasma chamber 100 mayalso be formed from a coated metal such as anodized aluminum or nickelplated aluminum. The plasma chamber 100 includes imbedded coolingchannels 102 for passing a fluid that controls the temperature of theplasma chamber 100.

As shown, a first 104 and a second high permeability magnetic core 106surround the plasma chamber 100. The magnetic cores 104, 106 are part ofthe transformer 12 of FIG. 1. As described in connection with FIG. 1,each of the first 104 and the second core 106 induce a potential insidethe chamber that forms a plasma which completes a secondary circuit ofthe transformer 12. Only one magnetic core is required to operate thetoroidal low-field plasma source.

Applicants have discovered that an inductively-driven toroidal low-fieldplasma source can be made with a metallic plasma chamber. Prior artinductively coupled plasma sources use plasma chambers formed fromdielectric material so as to prevent induced current flow from formingin the plasma chamber itself. The plasma chamber 100 of this inventionincludes at least one dielectric region that electrically isolates aportion of the plasma chamber 100 so that electrical continuity throughthe plasma chamber 100 is broken. The electrical isolation preventsinduced current flow from forming in the plasma chamber itself.

The plasma chamber 100 includes a first 108 and a second dielectricregion 110 that prevents induced current flow from forming in the plasmachamber 100. The dielectric regions 108, 110 electrically isolate theplasma chamber 100 into a first 112 and a second region 114. Each of thefirst 112 and the second region 114 is joined with a high vacuum seal tothe dielectric regions 108, 110 to form the plasma chamber 100. The highvacuum seal may be comprised of an elastomer seal or may be formed by apermanent seal such as a brazed joint. In order to reduce contamination,the dielectric regions 108, 110 may be protected from the plasma. Thedielectric regions 108, 110 may comprise a dielectric spacer separatingmating surface 116 of the plasma chamber 100, or may be a dielectriccoating on the mating surface 116.

In operation, a feed gas flows into an inlet 118. As described inconnection with FIG. 1, each of the first 104 and the second core 106induce a potential inside the plasma chamber 100 that forms a plasmawhich completes a secondary circuit of the transformer 12. Note thatonly one magnetic core is required to operate the toroidal low-fieldplasma source.

The use of metal or coated metal chambers in toroidal low-field plasmasources is advantageous because some metals are more highly resistant tocertain chemicals commonly used in plasma processing, such as fluorinebased gases. In addition, metal or coated metal chambers may have muchhigher thermal conductivity at much higher temperatures than dielectricchambers and, therefore, can generate much higher power plasmas.

FIG. 4 is a schematic representation of a dielectric spacer 150 suitablefor the dielectric regions illustrated in FIG. 3 that prevent inducedcurrent flow from forming in the plasma chamber. In this embodiment, ahigh vacuum seal 152 is formed outside the dielectric spacer 150. Thedielectric region is protected from the plasma by protruded chamber wall100.

FIG. 5 is a schematic representation of an ion beam source 200 includingan toroidal low-field plasma generator that embodies the invention. Theion beam source 200 may be used for numerous ion beam processingapplications including ion milling and ion implantation. The ion beamsource 200 includes toroidal low field plasma source 202 comprising themetallic plasma chamber 100 described in connection with FIG. 3. Theplasma chamber 100 includes a slit 204 for extracting ions generated bythe plasma out of the chamber 100. Accelerating electrodes 206accelerate the ions passing out of the chamber 100 with a predeterminedelectric field thereby forming an ion beam where the ions have apredetermined energy.

A mass-separating magnet 208 may be positioned in the path of theaccelerated ions to select a desired ion species. A second set ofaccelerating electrodes may be used to accelerate the desired ionspecies to a predetermined high energy. An ion lens may be used to focusthe high energy ion beam. A vertical 212 and a horizontal axis scanner214 may be used to scan the ion beam across a sample 216. A deflector218 may be used to separate the ion beam from any neutral particles sothat the ion beam impacts the sample 216 and the neutral particlesimpact a neutral trap 220.

FIG. 6 is a schematic block diagram of a solid state switching powersupply 250 that includes the one or more switching semiconductor devicesof FIG. 1. Applicants have discovered that switching semiconductordevices can be used to drive the primary winding of a power transformerthat couples electromagnetic energy to a plasma so as to form asecondary circuit of the transformer.

The use of a switching power supply in toroidal low-field plasma sourceis advantageous because switching power supplies are much less expensiveand are physically much smaller in volume and lighter in weight than theprior art RF and microwave power supplies used to power plasma sources.This is because switching power supplies do not require a line isolationcircuit or an impedance matching network.

The present invention can use any switching power supply configurationto drive current in the primary winding 18 (FIG. 1). For example, theswitching power supply 250 may include a filter 252 and a rectifiercircuit 254 that is coupled to a line voltage supply 256. An output 258of the filter 252 and the rectifier circuit 254 produces a DC voltagewhich is typically several hundred volts. The output 258 is coupled to acurrent mode control circuit 260.

The current mode control circuit 260 is coupled to a first 262, 262 aand a second isolation driver 264, 264 a. The first 262, 262 a and thesecond isolation driver 264, 264 a drives a first 266 and a second pairof switching transistors 268. The switching transistors may be IGBT orFET devices. The output of the first 266 and the second pair ofswitching transistors 268 may have numerous waveforms including asinusoidal waveform. The output of the switching transistors is coupledas shown by an inductor 267 in series with the primary winding andmagnetic core 269 to the toroidal plasma 270 which forms the transformersecondary.

Equivalents

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method for dissociating gases comprising: a)confining a gas in a plasma chamber at a pressure; b) providing atransformer having a magnetic core surrounding a portion of the chamberand having a primary winding; c) generating an AC current using a solidstate AC switching power supply comprising one or more switchingsemiconductor devices and having an output that is coupled to theprimary winding of the transformer; d) inducing an AC potential insidethe plasma chamber by supplying the current to the primary winding, theinduced AC potential forming a toroidal plasma that completes asecondary circuit of the transformer and that dissociates the gas; e)measuring the current supplied to the primary winding; f) comparing themeasured current to a predetermined setpoint representative of a desiredoperating condition; and g) adjusting one or more electrical parametersof the AC switching power supply to control the current supplied to theprimary winding.
 2. The method of claim 1 wherein the one or moreelectrical parameters are chosen from the group consisting of pulseamplitude, frequency, pulse width and phase.
 3. The method of claim 1wherein the output of the AC switching power supply is coupled, withoutusing an impedance matching network, to the primary winding of thetransformer and wherein the AC switching power supply supplies, withoutusing the impedance matching network, the current to the primarywinding.
 4. The method of claim 1 wherein the output of the AC switchingpower supply is coupled, without requiring a conventional impedancematching network, to the primary winding of the transformer and whereinthe AC switching power supply supplies, without requiring theconventional impedance matching network, the current to the primarywinding.
 5. The method of claim 1 wherein the output of the AC switchingpower supply is coupled, without using a conventional impedance matchingnetwork, to the primary winding of the transformer and wherein the ACswitching power supply supplies, without using the conventionalimpedance matching network, the current to the primary winding.
 6. Amethod for dissociating gases comprising: a) confining a gas in a plasmachamber at a pressure; b) providing a transformer having a magnetic coresurrounding a portion of the chamber and having a primary winding; c)generating an AC current using a solid state AC switching power supplycomprising one or more switching semiconductor devices and having anoutput that is coupled to the primary winding of the transformer; d)inducing an AC potential inside the plasma chamber by supplying thecurrent to the primary winding, the induced AC potential forming atoroidal plasma that completes a secondary circuit of the transformerand that dissociates the gas; e) measuring power in the toroidal plasma;f) comparing the measured power to a predetermined setpointrepresentative of a desired operating condition; and g) adjusting afrequency of the AC switching power supply to control a delivered to thetoroidal plasma.
 7. A method for dissociating gases comprising: a)confining a gas in a plasma chamber at a pressure; b) providing atransformer having a magnetic core surrounding a portion of the chamberand having a primary winding; c) generating a substantially sinusoidalcurrent using a solid state AC switching power supply comprising one ormore switching semiconductor devices and having an output that iscoupled to the primary winding of the transformer; and d) inducing an ACpotential inside the plasma chamber by supplying the substantiallysinusoidal current to the primary winding, the induced AC potentialforming a toroidal plasma that completes a secondary circuit of thetransformer and that dissociates the gas.
 8. An apparatus fordissociating gases comprising: a) a plasma chamber comprising a gas; b)a transformer having a magnetic core surrounding a portion of the plasmachamber and having a primary winding; c) a gas inlet for providing afeed gas to the plasma chamber; and d) a solid state AC switching powersupply comprising one or more switching semiconductor devices and havingan output that is coupled through a reactive element to the primarywinding of the transformer, the AC switching power supply supplying anAC current through the reactive element to the primary winding, thecurrent inducing an AC potential inside the plasma chamber that forms atoroidal plasma that completes a secondary circuit of the transformerand that dissociates the feed gas.
 9. The apparatus of claim 8 whereinthe AC switching power supply supplies the current to the primarywinding without the use of the conventional impedance matching network,and wherein the AC switching power supply supplies the current to theprimary winding without the use of the conventional impedance matchingnetwork.
 10. The apparatus of claim 8 wherein the AC switching powersupply supplying a substantially sinusoidal current.
 11. The apparatusof claim 8 wherein the reactive element is an inductor.
 12. Theapparatus of claim 8 wherein the plasma chamber comprises at least oneof a metallic material and dielectric material.
 13. The apparatus ofclaim 8 further comprising a circuit for measuring the current suppliedto the primary winding of the transformer, comparing the measuredcurrent to a predetermined setpoint representative of a desiredoperating condition, and adjusting one or more electrical parameters ofthe switching power supply to control the current supplied to theprimary winding.
 14. The apparatus of claim 8 wherein the one or moreelectrical parameters are chosen from the group consisting of pulseamplitude, frequency, pulse width and phase.
 15. The apparatus of claim8 further comprising a circuit for measuring a power in the toroidalplasma, comparing the measured power to a predetermined setpointrepresentative of a desired operating condition, and adjusting frequencyof the AC switching power supply to control a power delivered to thetoroidal plasma.